1. Field of the Invention
The present invention relates to a glass composition for an eye glass lens having a refractive index of 1.69 or higher, an Abbe number of 40 or higher, and a specific gravity of 3.2 or lower.
2. Description of the Prior Art
The correction of eye glass lens depends upon the refractive index and the curvatures of both surfaces of the lens but when the degree of myopia or hypermetropia advances, the thickness of the lens for correcting it increases at the outer edge portion and hence the appearance is spoiled and the lens becomes heavy resulting in an unpleasant feeling to the user. Therefore, a glass for an eye glass lens having a light weight and a high refractive index has been demanded and some glasses have been developed for these purposes as disclosed in, for example, U.S. Pat. No. 3,615,770. However, the glass disclosed therein has a small Abbe number of 30.1-31.5 and thus the lens has the disadvantage that the chromatic abberation of the outer edge portion thereof is large when the lens is used as an eye glass lens.
In particular, when the glass is used as an eye glass lens having a high diopter value, e.g., above .+-.10, the chromatic abberation becomes a serious problem.